Use the generated record in the thumbnail table instead

refs #29

The migration is assuming network indexing wasn't present before now,
and all existing files are local ones
Fix #48

This allows folders to be listed based on the amount of audio media,
video media, or both, instead of "any media"
Fix #46

ref #42

ref #42

Which stores whether the thumbnail was generated by the medialibrary
itself or not.

Drop AlbumArtistRelation table and only use the AlbumTrack table.
This reduces the database size, simplifies the code, and increase the
similarity between unittests & actual metadata parsing handling

The album now relies on the media is_present flag to compute its overall
presence.
This drops a layer of indirrection when (un)plugging a removable device

Bypass those tables and directly propagate the presence state in the
media

This can be used to avoid full table scans when knowing if a media is
present in a playlist or not

So that we can use it for vacuuming old external media

Use External instead, since that's what they are

Fix #36

We were leaving potentially scheduled parser tasks behind

This is now handled by a specific task table in DB

6 to 7 migration will remove all albums & artists anyway, so there's now
no need to update their values during 5 -> 6.
The test is removed as regardless of the migration, it would now yeild
success due to the inversion of the condition when fetching present
entities (is_present = 1 became is_present != 0)

This reverts commit 0fc6b3bb.